The present invention relates to a superconductor structure. Specifically, the present invention relates to a superconductor structure comprising a barrier layer of material between a superconductive material and a substrate.
Superconductors refer to a class of compounds which exhibit a complete disappearance of electrical resistance and repel the magnetic field from the inner volume of the material (Meisner effect) under certain conditions. While various superconductors have been known for a number of years, practical applications for such superconductors have been limited due to a variety of factors. For example, a number of compounds which exhibit superconductivity only exhibit such superconductivity at temperatures near absolute zero. Due to the difficulty of maintaining such low temperatures, the practical applications of such superconductors have been severely limited.
Recently, a massive research effort has been conducted in an attempt to identify compositions exhibiting superconductivity which compositions are capable of exhibiting superconductivity at temperatures significantly above absolute zero.
One type of compound which has been found to exhibit superconductivity at temperatures above absolute zero are certain ceramic compositions comprising copper and oxygen, beneficially comprising copper, oxygen and at least one element selected from the group consisting of bismuth, strontium, calcium, thallium, indium and the rare earth elements and particularly compounds having the general formula: EQU Re.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x
wherein Re represents a rare earth element selected from the group consisting of Er, Gd, Y, Tm, Sm, Eu, Dy, Ho, Yb, Nd, and Lu and 0.ltoreq.x.ltoreq.0.5. When x is within the described ranges, the oxygen content of the composition is between about 6.5 and 7.0 per unit formula. The ceramic compositions represented by the above described formula exhibit superconductivity at a temperature between 90-100K. While 90-100K is still a very low temperature, maintaining a superconductor at such a temperature is greatly simplified when compared to maintaining a superconductor at or near absolute zero. For example, a temperature of 90-100K can be maintained by employing liquid nitrogen as a coolant. Thus, the ceramic superconducting compounds described above are attractive possibilities in the attempt to adapt superconducting materials for practical applications.
Despite the advantage of demonstrating superconductivity at higher temperatures, the ceramic compositions described above suffer from various other drawbacks. For example, it is known that in order for the described ceramic compositions to exhibit superconductivity, it is important to maintain the stoichiometry of the various elements within very specific ratios, such as those described by the above formula. If the stoichiometry of the compositions is not maintained, the compositions tend to deteriorate and eventually lose their superconductivity.
Additionally, the described ceramic superconductive compositions tend to interact with other materials with which they are in contact. Such interaction between the ceramic superconductive materials and other materials with which they come in contact generally results in the loss of stoichiometry in the materials, thus resulting in the decomposition and eventual loss of superconductivity.
For example, it is often desirable to employ a relatively thin layer of superconductive material in a device. In those instances wherein it is desired to employ a relatively thin layer of superconductive material, it is often necessary to use a substrate as a support for the thin layer of superconductive material.
Unfortunately, it has been discovered that when it is desired to apply a relatively thin layer of superconductive material to a substrate, interaction can occur between the material from which the substrate is formed and the layer of superconductive material. As a result of this interaction, at least a portion of the layer of superconductive material closest to the substrate will lose the stoichiometry desired to impart the superconductivity to the superconductive layer. In effect, and as result of this intraction, a multi-phase layer is formed.
Various attempts have been made to avoid or compensate for the interaction between the substrate and superconductive material layer. For example, it has been suggested that the substrate be selected so that there is a minimum amount of interaction between the substrate layer and the layer of superconductive material. For example, one of the most common substrates employed in the electronics industry is alumina (Al.sub.2 O.sub.3). Unfortunately, alumina has been found to be relatively reactive with ceramic superconductive materials. Accordingly, it has been proposed that strontium titanate (SrTiO.sub.3) be employed as the substrate material. This is because SrTiO.sub.3 has been found to be relatively less reactive with the ceramic superconductive materials than alumina and allows for the epitaxial growth of thin films of superconductive materials according to the formula Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x.
Unfortunately, SrTiO.sub.3 is more expensive and less readily available for use as a substrate and is still somewhat reactive with ceramic superconductive materials. Additionally, SrTiO.sub.3 has a very high dielectric constant which limits its use in high frequency applications. Many of the proposed applications for superconductive materials would benefit if it were possible to employ the substrates normally employed with electronic devices in forming superconductor structures.
Moreover, ceramic superconductive materials comprising copper and oxygen have been found to possess relatively poor adhesion to many substrates. For example, when a ceramic superconductive material comprising copper and oxygen is applied to an alumina substrate at a relatively low temperature, the ceramic superconductive layer will often peel off the substrate. When applied at a higher temperature, peeling may be avoided but the higher temperature enhances the deleterious interaction between the substrate and the superconductive layer.